Alpha, beta, beta-trifluorostyrene-based composite membranes

ABSTRACT

A membrane electrode assembly comprises first and second electrodes and a composite membrane interposed therebetween. The composite membrane comprises a porous substrate impregnated with a polymeric composition comprising polymerized α,β,β-trifluorostyrene monomeric units. Where the polymeric composition includes ion-exchange moieties, the resultant membrane electrode assembly is useful in electrochemical applications, particularly as a membrane electrode assembly in electrochemical fuel cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/901,269 filed Jul. 9, 2001, now U.S. Pat. No.______issued______. The ′269 application is, in turn, a continuation of U.S.patent application Ser. No. 09/441,181 filed Nov. 15, 1999, now U.S.Pat. No. 6,258,861 issued Jul. 10, 2001. The ′181 application is, inturn, a continuation of U.S. patent application Ser. No. 09/186,449filed Nov. 5, 1998, now U.S. Pat. No. 5,985,942 issued Nov. 16, 1999.The ′449 application is, in turn, a continuation of U.S. patentapplication Ser. No. 08/583,638 filed Jan. 5, 1996, now U.S. Pat. No.5,834,523 issued Nov. 10, 1998. The ′638 application is, in turn, acontinuation-in-part of U.S. patent application Ser. No. 08/442,206filed May 16, 1995, now U.S. Pat. No. 5,498,639 issued Mar. 12, 1996.The ′206 application is, in turn, a continuation of U.S. patentapplication Ser. No. 08/124,924 filed Sep. 21, 1993, now U.S. Pat. No.5,422,411 issued Jun. 6, 1995, entitled “Trifluorostyrene andSubstituted Trifluorostyrene Copolymeric Compositions and Ion-exchangeMembranes Formed Therefrom”. The ′269, ′181, ′449, ′638, ′206, and ′924applications, each of which is hereby incorporated by reference hereinin its entirety, describe polymeric compositions, as well as compositemembranes thereof, derived from copolymers of α,β,β-tri-fluorostyrenewith a variety of substituted α,β,β-trifluorostyrenes. Thesecompositions are suitable for use as membranes, particularly asion-exchange membranes.

[0002] This application is also related to U.S. patent application Ser.No. 08/480,098 filed Jun. 6, 1995, now U.S. Pat. No. 5,602,185 issuedFeb. 11, 1997, U.S. patent application Ser. No. 08/575,349 filed Dec.20, 1995, now U.S. Pat. No. 5,684,192 issued Nov. 4, 1997, both entitled“Substituted Trifluorostyrene Compositions”, and U.S. patent applicationSer. No. 08/768,615 filed Dec. 18, 1996, now U.S. Pat. No. 5,773,480issued Jun. 30, 1998, entitled “Trifluorostyrene and SubstitutedTrifluorostyrene Copolymeric Compositions and Ion-exchange MembranesFormed Therefrom”, which is a continuation-in-part of the ′098application. The ′098, ′349 and ′615 applications, each of which is alsohereby incorporated by reference herein in its entirety, describecopolymers of α,β,β-tri-fluorostyrene and substitutedα,β,β-trifluorosty-renes, including sulfonyl fluoride substitutedα,β,β-trifluorostyrene monomeric units that are conveniently hydrolyzedto produce polymeric compositions with ion-exchange moieties. Thisapplication is further related to U.S. patent application Ser. No.08/482,948 filed Jun. 7, 1995, entitled “Copolymeric Compositions ofTrifluorostyrene, Substituted Trifluorostyrene and Substituted Ethylene,and Ion-exchange Membranes Formed Therefrom”. The ′948 application,which is also hereby incorporated by reference herein in its entirety,describes copolymers of α,β,β-tri-fluorostyrene and substitutedα,β,β-trifluorostyrenes with substituted ethylene monomeric units. Thesecompositions are suitable for use as membranes, particularly asion-exchange membranes.

FIELD OF THE INVENTION

[0003] This invention relates generally to composite membranescomprising a porous substrate and a polymeric composition comprisingvarious combinations of α,β,β-trifluorostyrene, substitutedα,β,β-trifluorostyrene and ethylene-based monomeric units. Where thepolymeric composition includes ion-exchange moieties, the resultantcomposite membranes are useful in electrochemical applications,particularly as membrane electrolytes in electrochemical fuel cells.

BACKGROUND OF THE INVENTION

[0004] Dense films can be obtained from solutions ofpoly-α,β,β-trifluorostyrene. However, the brittleness of these filmsgreatly limits their application. Films obtained from some sulfonatedpoly-α,β,β-trifluorostyrenes can be used as ion-exchange membranes.However, such films often have unfavorable mechanical properties whenwet, and are known to be very brittle in the dry state (see, forexample, Russian Chemical Reviews, Vol. 59, p. 583 (1988)). Such filmsare of little practical use in fuel cells due to their poor physicalproperties. Some improvements in mechanical properties have beenachieved by blending sulfonated poly-α,β,β-trifluorostyrene withpolyvinylidene fluoride and triethyl phosphate plasticizer, but thesefilms remained unsatisfactory for application in electrochemical cells(see Fuel Cell Handbook, A. J. Appleby, published by Van NostrandReinhold, p. 286 (1989)).

[0005] U.S. Pat. No. 5,422,411 and the related patent applicationsmentioned above describe various polymeric compositions incorporatingsubstituted α,β,β-trifluorostyrenes and some cases further incorporatingsubstituted ethylenes. Typically these compositions, as membranes,possess favorable mechanical properties compared topoly-α,β,β-trifluorostyrene and sulfonated poly-α,β,β-trifluorostyrene,although some of the membranes have a tendency to become brittle in thefully dehydrated state, depending, for example, on the equivalentweight. This effect is most apparent at equivalent weights belowapproximately 380 g/mol. Ion-exchange membranes derived from thesepolymeric compositions are suitable for many applications, including usein electrochemical applications, such as fuel cells.

[0006] For ease of handling, for example, in the preparation of membraneelectrode assemblies for use in electrochemical fuel cells, themechanical strength of the membrane in the dry state is important. Inelectrochemical applications, such as electrolytic cells and fuel cells,the dimensional stability (changes in the dimensions of the membrane dueto changes in the degree of hydration) of the membrane during operationis also important. However, to improve performance, it is generallydesirable to reduce membrane thickness and to decrease the equivalentweight (thereby increasing the water content) of the membraneelectrolyte, both of which tend to decrease both the mechanical strengthin the dry state and the dimensional stability in the wet state. One wayto improve mechanical strength and dimensional stability in ionomericmembranes is through use of a substrate or support material, to give acomposite membrane. The substrate is selected so that it impartsmechanical strength and dimensional stability to the membrane. Thesubstrate material can be combined with the membrane polymeric materialto form a composite membrane in a variety of ways. For example, ifpossible, an unsupported membrane can be preformed and then laminated tothe porous substrate. Alternatively, a solution of the polymer can beimpregnated into the porous substrate material, and the compositemembrane subsequently dried. The formation of composite membranes viaimpregnation provides a more intimate contact between the twocomponents, thus giving advantages over standard lamination approaches.

[0007] Composite ion-exchange membranes prepared by impregnatingcommercially available porous polytetrafluoroethylene film (Gore-tex®)with Nafion®, a perfluorosulfonate ionomer, have been described inJournal of the Electrochemical Society, Vol. 132, pp. 514-515 (1985).The major goal in the study was to develop a composite membrane with thedesirable chemical and mechanical features of Nation®, but which couldbe produced at low cost. Indeed, based on the polymer loadings necessaryto produce these composite membranes, they are a low cost alternative tothe costly perfluorosulfonic acid membranes. As indicated above,however, these perfluorosulfonate ionomers are known to form membranessuitable for use in electrochemical applications without the use of asubstrate.

[0008] It has been discovered that polymers which have a tendency tobecome brittle in the dehydrated state can be rendered mechanicallystable, even in the fully dehydrated state, by impregnation intosuitable substrates.

[0009] Furthermore, it has been discovered that even polymers which arepoor film formers, or polymers which form films with mechanicalproperties and dimensional stability which would preclude their use inelectrochemical and other applications, can be made into compositemembranes through incorporation into a suitable substrate. The resultingcomposite membranes have the desired physical properties for use in awide range of applications.

SUMMARY OF THE INVENTION

[0010] In one aspect of the present invention, a composite membranecomprises a porous substrate impregnated with a polymeric compositioncomprising α,β,β-trifluorostyrene monomeric units.

[0011] In another aspect, a composite membrane comprises a poroussubstrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units. Substitutedα,β,β-trifluorostyrene monomeric units have at least one non-hydrogensubstituent on the aromatic ring. In a preferred embodiment, thepolymeric composition comprises at least two different substitutedα,β,β-trifluorostyrene monomeric units.

[0012] In a first embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingα,β,β-trifluorostyrene monomeric units, the polymeric compositionfurther comprises ethylene monomeric units, the polymeric compositionderived from a copolymerization reaction involving at least ethylene andα,β,β-trifluorostyrene.

[0013] In a second embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingα,β,β-trifluorostyrene monomeric units, the polymeric compositionfurther comprises partially fluorinated ethylene monomeric units, thepolymeric composition derived from a copolymerization reaction involvingat least α,β,β-trifluorostyrene and, for example, CH₂═CHF, CHF═CHF,CF₂═CH₂, or CF₂═CHF.

[0014] In a third embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingα,β,β-trifluorostyrene monomeric units, the polymeric compositionfurther comprises tetrafluoroethylene monomeric units, the polymericcomposition derived from a copolymerization reaction involving at leasttetrafluoroethylene and α,β,β-trifluorostyrene.

[0015] In a fourth embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingα,β,β-trifluorostyrene monomeric units, the polymeric compositionfurther comprises:

[0016] where m is an integer greater than zero; Y is selected from thegroup consisting of chlorine, bromine, iodine, C_(x)H_(y)F_(z) (where xis an integer greater than zero and y+z=2x+1), O—R (where R is selectedfrom the group consisting of C_(x)H_(y)F_(z) (where x is an integergreater than zero and y+z=2x+1) and aryls), CF═CF₂, CN, COOH and CO₂R¹(where R¹ is selected from the group consisting of perfluoroalkyls,aryls, and NR²R³ where R² and R³ are selected from the group consistingof hydrogen, alkyls and aryls).

[0017] In a fifth embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingα,β,β-trifluorostyrene monomeric units, the polymeric compositionfurther comprises styrene monomeric units, the polymeric compositionderived from a copolymerization reaction involving at least styrene andα,β,β-trifluorostyrene.

[0018] In a sixth embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingα,β,β-trifluorostyrene monomeric units, the polymeric compositionfurther comprises substituted styrene monomeric units, the polymericcomposition derived from a copolymerization reaction involving at leasta substituted styrene and α,β,β-trifluorostyrene. Substituted styrenemonomeric units have at least one non-hydrogen substituent on thearomatic ring.

[0019] In a first embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition comprises:

[0020] where m is an integer greater than zero. In a further embodimentthe polymeric composition comprises:

[0021] where m is an integer greater than zero, and at least one of n, pand q is an integer greater than zero; A₁, A₂ and A₃ are selected fromthe group consisting of hydrogen, halogens, C_(x)H_(y)F_(z) (where x isan integer greater than zero and y+z=2x+1), CF═CF₂, CN, NO₂ and OH, O—R(where R is selected from the group consisting of alkyls andperfluoroalkyls and aryls). In a still further embodiment, the groupfrom which A₁, A₂ and A₃ are selected further consists of SO₃H, PO₂H₂,PO₃H₂, CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂, NR₃ ⁺(where R is selectedfrom the group consisting of alkyls, perfluoroalkyls and aryls) andCH₂NR₃ ⁺(where R is selected from the group consisting of alkyls,perfluoroalkyls and aryls), and at least one of A₁, A₂ and A₃ isselected from the group consisting of SO₃H, PO₂H₂, PO₃H₂, CH₂PO₃H₂,COOH, OSO₃H, OPO₂H₂, OPO₃H₂, NR₃ ⁺(where R is selected from the groupconsisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃ ⁺(where R isselected from the group consisting of alkyls, perfluoroalkyls andaryls).

[0022] In a second embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition comprises:

[0023] where at least one of n, p and q is an integer greater than zero;A₁, A₂ and A₃ are selected from the group consisting of CF═CF₂, CN, NO₂and OH, O—R (where R is selected from the group consisting ofC_(x)H_(y)F_(z) (where x is an integer greater than three and y+z=2x+1)and aryls).

[0024] In a third embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition comprises:

[0025] where m is an integer greater than zero.

[0026] In a fourth embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition comprises:

[0027] where m is an integer greater than zero; X is selected from thegroup consisting of PO₂H₂, PO₃H₂, CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂,OArSO₃H where Ar is an aryl, NR₃ ⁺(where R is selected from the groupconsisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃ ⁺(where R isselected from the group consisting of alkyls, perfluoroalkyls andaryls).

[0028] In a fifth embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition comprises:

[0029] where m is an integer greater than zero, and at least one of n, pand q is an integer greater than zero; X is selected from the groupconsisting of SO₃H, PO₂H₂, PO₃H₂, CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂,OArSO₃H where Ar is an aryl, NR₃ ⁺(where R is selected from the groupconsisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃ ⁺(where R isselected from the group consisting of alkyls, perfluoroalkyls and aryls); A₁, A₂ and A₃ are selected from the group consisting of halogens,C_(x)H_(y)F_(z) (where x is an integer greater than zero and y+z=2x+1),CF═CF₂, CN, NO₂ and OH, O—R (where R is selected from the groupconsisting of alkyls and perfluoroalkyls and aryls). In a furtherembodiment, the group from which A₁, A₂ and A₃ are selected furtherconsists of hydrogen. In a still further embodiment, the group fromwhich A₁, A₂ and A₃ are selected further consists of SO₃H, PO₂H₂, PO₃H₂,CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂, NR₃ ⁺(where R is selected fromthe group consisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃⁺(where R is selected from the group consisting of alkyls,perfluoroalkyls and aryls), and at least one of A₁, A₂ and A₃ isselected from the group consisting of SO₃H, PO₂H₂, PO₃H₂, CH₂PO₃H₂,COOH, OSO₃H, OPO₂H₂, OPO₃H₂, NR₃ ⁺(where R is selected from the groupconsisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃ ⁺(where R isselected from the group consisting of alkyls, perfluoroalkyls andaryls).

[0030] In a sixth embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition comprises:

[0031] where m is an integer greater than zero; B and D are selectedfrom the group consisting of hydrogen, SO₂F, SO₃H, PO₂H₂, PO₃H₂,CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂, NR₃ ⁺(where R is selected fromthe group consisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃⁺(where R is selected from the group consisting of alkyls,perfluoroalkyls and aryls). In a further embodiment, the polymericcomposition comprises:

[0032] where m is an integer greater than zero, and at least one of n, pand q is an integer greater than zero; B and D are selected from thegroup consisting of hydrogen, SO₂F, SO₃H, PO₂H₂, PO₃H₂, CH₂PO₃H₂, COOH,OSO₃H, OPO₂H₂, OPO₃H₂, NR₃ ⁺(where R is selected from the groupconsisting of alkyls, perfluoroalkyls and aryls) and CH₂NR₃ ⁺(where R isselected from the group consisting of alkyls, perfluoroalkyls and aryls); A₁, A₂ and A₃ are selected from the group consisting of hydrogen,SO₂F, halogens, C_(x)H_(y)F_(z) (where x is an integer greater than zeroand y+z=2x+1), CF═CF₂, CN, NO₂ and OH, O—R (where R is selected from thegroup consisting of alkyls and perfluoroalkyls and aryls). In a stillfurther embodiment, the group from which A₁, A₂ and A₃ are selectedfurther consists of SO₃H, PO₂H₂, PO₃H₂, CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂,OPO₃H₂, NR₃ ⁺(where R is selected from the group consisting of alkyls,perfluoroalkyls and aryls) and CH₂NR₃ ⁺(where R is selected from thegroup consisting of alkyls, perfluoroalkyls and aryls), and at least oneof A₁, A₂ and A₃ is selected from the group consisting of SO₃H, PO₂H₂,PO₃H₂, CH₂PO₃H₂, COOH, OSO₃H, OPO₂H₂, OPO₃H₂, NR₃ ⁺(where R is selectedfrom the group consisting of alkyls, perfluoroalkyls and aryls) andCH₂NR₃ ⁺(where R is selected from the group consisting of alkyls,perfluoroalkyls and aryls). In preferred embodiments B is hydrogen.

[0033] In a seventh embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition further comprises ethylene monomeric units.

[0034] In an eighth embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition further comprises partially fluorinated ethylene monomericunits, the polymeric composition derived from a copolymerizationreaction involving, for example, CH₂═CHF, CHF═CHF, CF₂═CH₂, or CF₂═CHF.

[0035] In a ninth embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition further comprises tetrafluoroethylene monomeric units.

[0036] In a tenth embodiment of a composite membrane comprising a poroussubstrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition further comprises:

[0037] where m is an integer greater than zero; Y is selected from thegroup consisting of chlorine, bromine, iodine, C_(x)H_(y)F_(z) (where xis an integer greater than zero and y+z=2x+1), O—R (where R is selectedfrom the group consisting of C_(x)H_(y)F_(z) (where x is an integergreater than zero and y+z =2x+1) and aryls), CF═CF₂, CN, COOH and CO₂R¹(where R¹ is selected from the group consisting of alkyls,perfluoroalkyls, aryls, and NR²R³ where R² and R³ are selected from thegroup consisting of hydrogen, alkyls and aryls).

[0038] In an eleventh embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition further comprises styrene monomeric units.

[0039] In a twelfth embodiment of a composite membrane comprising aporous substrate impregnated with a polymeric composition comprisingsubstituted α,β,β-trifluorostyrene monomeric units, the polymericcomposition further comprises substituted styrene monomeric units.Substituted styrene monomeric units have at least one non-hydrogensubstituent on the aromatic ring.

[0040] In the aspects and embodiments described above, the substrate ispreferably a porous film or sheet material. For electrochemicalapplications, for example, preferred porous substrates comprise, orconsist essentially of, porous polyolefins. Preferred polyolefins arepolyethylene and polypropylene. Particularly preferred substratescomprise, or consist essentially of, porous polytetrafluoroethylene,also known as expanded polytetrafluoroethylene.

[0041] In a preferred aspect, a composite membrane comprises a poroussubstrate impregnated with a polymeric composition comprising:

[0042] where m and n are integers greater than zero and A₁ is selectedfrom the group consisting of fluorine, CF₃ and para-phenoxy. In afurther embodiment of this preferred aspect, the group from which A₁ isselected further consists of hydrogen.

[0043] In another preferred aspect, a composite membrane comprises aporous substrate impregnated with a polymeric composition comprising:

[0044] where m, n, and p are integers greater than zero and A₁ and A₂are selected from the group consisting of hydrogen, fluorine, CF₃, andpara-phenoxy.

[0045] In another preferred aspect, a composite membrane comprises aporous substrate impregnated with a polymeric composition comprising:

[0046] where m and n are integers greater than zero and X is selectedfrom the group consisting of para-SO₂F, meta-SO₃H and para-SO₃H.

[0047] In yet another preferred aspect, a composite membrane comprises aporous substrate impregnated with a polymeric composition comprising:

[0048] where m and q are integers greater than zero, n and p are zero oran integer greater than zero; X is selected from the group consisting ofpara-SO₂F, meta-SO₃H and para-SO₃H; and A₁ and A₂ are selected from thegroup consisting of hydrogen, fluorine, CF₃, and para-phenoxy. In afurther embodiment of this preferred aspect, n is an integer greaterthan zero.

[0049] In still another preferred aspect, a composite membrane comprisesa porous substrate impregnated with a polymeric composition comprising:

[0050] where m and q are integers greater than zero, n and p are zero oran integer greater than zero; X is selected from the group consisting ofpara-SO₂F, meta-SO₃H and para-SO₃H; and A₁ and A₂ are selected from thegroup consisting of hydrogen, fluorine, CF₃, and para-phenoxy. In afurther embodiment of this preferred aspect, n is an integer greaterthan zero.

[0051] In the aspects and embodiments described above, the polymericcompositions can consist essentially of the described monomeric units.

[0052] In all of the above preferred aspects, preferably the poroussubstrate comprises polytetrafluoroethylene. A preferred poroussubstrate consists essentially of polytetrafluoroethylene.

[0053] In the aspects and embodiments described above, the A₁, A₂, A₃substituents may be further elaborated by known means such as, forexample, by hydrolysis of the CN group to form COOH or by reduction withcommon reducing agents (such as, for example, Raney nickel) to form aprimary amine, thereby transforming the A₁, A₂ and A₃ substituents intoion-exchange moieties. The resulting polymeric composition may thuscomprise one or more type of ion-exchange moiety, and may also compriseboth cation-exchange and anion-exchange moieties.

[0054] The term “monomeric unit” as used herein indicates that thepolymeric composition contains the described fragment or unit, and isobtained by a polymerization reaction involving the correspondingunsaturated monomer.

[0055] The substituents on the aromatic rings (including, for example,A₁, A₂, A₃, X, B and D) may each be located in the ortho, meta or parapositions, as indicated in the formulas wherein the chemical bond drawnfor the substituents intersects the aromatic ring. In preferred aspectsof the described embodiments, the substituents are in the meta or parapositions.

[0056] As used herein, the term “aryl” refers to a substituted orunsubstituted phenyl group. The formula C_(x)H_(y)F_(z) (where x is aninteger greater than zero and y+z=2x+1) is used to indicate alkyl,perfluoroalkyl or partially fluorinated alkyl groups.

[0057] In accordance with convention in the art, the above chemicalformulas for polymeric compositions containing more than two monomericunits (where at least three of m, n, p and q are greater than zero) areintended to indicate that the monomeric units are present in thepolymeric composition, but are not limited to the particular order inwhich the monomeric units are set forth in each general formula. Forexample, random linear copolymers, alternating copolymers and linearblock copolymers, formed from the indicated monomeric units, arecontemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058]FIG. 1 is a plot of cell voltage as a function of current density(expressed in amperes per square foot or “ASF”) in an electrochemicalfuel cell employing a composite membrane of porous polyethyleneimpregnated with a sulfonated copolymer of α,β,β-trifluorostyrene andm-trifluoromethyl-α,β,β-trifluorostyrene, as the proton exchangemembrane.

[0059]FIG. 2 is a plot of cell voltage as a function of current densityin an electrochemical fuel cell employing a composite membrane, preparedby impregnation of porous polyethylene with sulfonatedpoly-α,β,β-trifluorostyrene, as the proton exchange membrane.

[0060]FIG. 3 is a plot of cell voltage as a function of current densityin an electrochemical fuel cell employing a composite membrane, preparedby impregnation of porous polyethylene with a copolymer ofα,β,β-trifluorostyrene, m-trifluoromethyl-α,β,β-trifluorostyrene andp-sulfonyl fluoride-α,β,β-trifluorostyrene, and subsequent hydrolysis,as the proton exchange membrane.

[0061]FIG. 4 is a plot of cell voltage as a function of current densityin an electrochemical fuel cell employing a composite membrane ofexpanded polytetrafluoroethylene impregnated with a sulfonated copolymerof α,β,β-trifluorostyrene and m-trifluoromethyl-α,β,β-trifluorostyrene,as the proton exchange membrane.

[0062]FIG. 5 is a plot of cell voltage as a function of current densityin an electrochemical fuel cell employing a composite membrane ofexpanded polytetrafluoroethylene impregnated with a low equivalentweight sulfonated copolymer of α,β,β-trifluorostyrene andm-trifluoromethyl-α,β,β-trifluorostyrene, as the proton exchangemembrane.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0063] Methods for preparing the polymeric compositions described hereinare described in the related applications or will be apparent to thoseskilled in the art.

[0064] The preferred substrate material is dependent on the applicationin which the composite membrane is to be used. The substrate materialpreferably has good mechanical properties, is chemically and thermallystable in the environment in which the composite membrane is to be used,is tolerant of the solvent used for impregnation, and in mostapplications is preferably flexible. For example, the porous substratecan be a woven or nonwoven fabric or cloth, or can be made of paper,fiber glass, cellulosics or a ceramic material. Preferred substrates forelectrochemical applications are porous polymeric materials. Preferredpolymeric materials are, for example, hydrocarbons such as porouspolyolefins, especially polyethylene and polypropylene. In someapplications, a perfluorinated polymeric substrate may be preferred, forexample, a preferred substrate material, when the composite membrane isto be used in an electrochemical fuel cell, is porouspolytetrafluoroethylene, also known as expanded polytetrafluoroethylene.Porous polyolefins and polytetrafluoroethylenes typically have excellentmechanical strength, flexibility and do not swell in water.Polytetrafluoroethylene offers additional advantages in that it is alsochemically inert, and porous polytetrafluoroethylene films withdifferent characteristics are commercially available from varioussources. It may be possible to obtain or prepare other suitable porouspolymeric substrates from, such as, for example, polyvinylidene fluorideor polysulfones. Copolymeric substrates such as, for example,poly(ethylene-co-tetrafluoroethylene) andpoly(tetrafluoroethylene-co-hexafluoropropylene), may also be used.

[0065] The degree of porosity, pore size and thickness of the substrateused in the composite membrane can be selected to suit the application.For use of the composite membrane as an electrolyte in anelectrochemical fuel cell, the substrate thickness is preferably 10-200μm, and more preferably 25-50 μm, the preferable average pore diameteris 0.1-1.0 μm, and the preferable porosity is 50-98%, more preferably75-90%.

[0066] Depending on the application the resultant composite membrane maybe gas permeable or gas impermeable. The loading of the polymericcomposition on the substrate can be varied in order to control theporosity of the resultant composite membrane. For fuel cellapplications, the composite membrane is preferably substantially gasimpermeable, thus the degree of impregnation and loading is such thatthe porosity of the composite membrane is reduced essentially to zero.

[0067] In a method for preparing composite membranes, the polymericcomposition is dissolved in a solvent, typically an organic solvent, toform a solution. The solvent used will depend, for example, on both thenature of the polymeric composition and the substrate. For impregnationof porous polyolefins with the type of polymeric compositions describedherein, suitable solvents include N,N-dimethylformamide,N-methyl-pyrrolidone, dimethylsulfoxide and N,N-dimethyl-acetamide. Whenpolytetrafluoroethylene is the substrate, an alcohol or mixture ofalcohols (chosen, for example, from methanol, ethanol and propan-2-ol)is often the preferred solvent. The concentration of the solution willdepend on the loading desired, and whether the composite membrane is tobe porous or not. For example, if the composite membrane is to be gaspermeable a lower concentration is generally preferred.

[0068] The porous substrate is then impregnated, for example, byconstraining the substrate in a frame and dipping or soaking it in thesolution. The contact time is dependent on the viscosity and percentagesolids of the solution. Other techniques known in the art, such asultrasonication, may be used to facilitate impregnation. Also, multipleimpregnations, possibly with different polymeric compositions, may bedesirable for some applications. The substrate is then removed from thesolution and the composite membrane dried preferably in a humiditycontrolled atmosphere (generally at less than or equal to 2% relativehumidity) at above ambient temperatures.

[0069] If the composite membrane includes proton-exchange moieties andis to be used in, for example, a proton-exchange membrane fuel cell, itis removed from the frame, treated with 1 M hydrochloric acid and washedwith deionized water prior to use.

[0070] The means by which the process described above could be modifiedfor impregnation of non-membrane substrates, and also for a continuouscomposite membrane manufacturing process will be apparent to thoseskilled in the art.

[0071] In the preparation of composite ion-exchange membranes, theion-exchange moieties can be:

[0072] (a) present in the polymeric composition prior to itsimpregnation into the substrate; or

[0073] (b) introduced post-impregnation through further reaction of thepolymeric composition on the substrate; or

[0074] (c) introduced via conversion of precursor groups, present in thepolymeric composition, after impregnation.

[0075] If the ion-exchange moieties are to be introduced via apost-impregnation conversion or reaction, the substrate needs toselected such that it can withstand the post-impregnation treatmentstep. For example, in post-impregnation introduction of ion-exchangemoieties, the ion-exchange moieties may be introduced into polymericcompositions containing unsubstituted α,β,β-tri-fluorostyrene units (socalled “base polymers”) via aromatic substitution of at least a portionof those units, after preparation of a composite membrane. For example,pendant unsubstituted phenyl rings in the composite membrane can beconveniently sulfonated (see U.S. Pat. No. 5,422,411) to produce acomposite cation-exchange membrane. Similarly, such pendantunsubstituted phenyl rings may be phosphorylated, carboxylated,quaternary-aminoalkylated or chloromethylated, and further modified toinclude —CH₂PO₃H₂, —CH₂NR₃ ⁺where R is an alkyl, or —CH₂NAr₃ ⁺(where Aris a substituted or unsubstituted phenyl group) and other substituents,to provide cation-exchange or anion-exchange composite membranes.Further still, the pendent phenyl moiety may contain a hydroxyl groupwhich can be elaborated by known methods to generate —OSO₃H, —OPO₂H₂ and—OPO₃H₂ cation-exchange sites on the composite membrane.

[0076] The approach in which the ion-exchange functionality isintroduced post-impregnation via conversion of a precursor using simplepost-impregnation procedure, such as hydrolysis, can be advantageous.For example, composite membranes comprising polymers containing sulfonylfluoride moieties (—SO₂F) can be hydrolyzed to generate —SO₃Hcation-exchange sites. In a typical hydrolysis reaction, the sulfonylfluoride is converted to the free sulfonic acid functionality bytreatment of the composite membrane with concentrated aqueous alkalimetal hydroxide at elevated temperatures. This and other procedures forthe hydrolysis of —SO₂F to —SO₃H are well-known to those skilled in theart. The latter approach to the introduction of —SO₃H moieties offersadvantages over sulfonation of a base polymer in the composite membrane.For example, it permits greater control over the ion-exchange capacityof the resultant composite membrane.

[0077] Membranes including sulfonyl fluoride substitutedα,β,β-trifluorostyrene monomeric units are described in a relatedapplication. Unsupported membranes containing a significant proportionof sulfonyl fluoride substituted α,β,β-trifluorostyrene monomeric unitscan be very fragile. The mechanical properties of these precursorion-exchange membranes can be significantly enhanced throughincorporation into a porous substrate.

[0078] It may be advantageous to introduce ion-exchange moieties afterpreparation of the composite membranes, as described in (b) and (c)above. For example, in electrochemical applications where the preferredsubstrates are typically hydrophobic, the preparation of a compositemembrane by first impregnating the substrate with a solution of anon-ionic polymer which is also essentially hydrophobic may lead to morefacile and improved impregnation.

[0079] The following examples are for purposes of illustration and arenot intended to limit the invention. Examples 1-3 describe thepreparation of composite ion-exchange membranes in which porous, highdensity polyethylene is used as the substrate. Examples 4 and 5 describethe preparation of composite ion-exchange membranes in which expandedpolytetrafluoroethylene is used as the porous substrate. In Examples 1,2, 4 and 5 the ion-exchange moieties were present in the polymericcomposition prior to its impregnation into the substrate. In Example 3the ion-exchange moiety was generated by hydrolysis of sulfonyl fluoridemoieties after preparation of the composite membrane. Example 6 setsforth the procedure used to test the composite ion-exchange membranes,prepared as described in Examples 1-5, as membrane electrolytes in anelectrochemical fuel cell.

[0080] EXAMPLE 1

[0081] Porous Polyethylene Impregnated with a Sulfonated Copolymer ofα,β,β-trifluorostyrene and m-trifluoromethyl-α,β,β-trifluorostyrene

[0082] (Composite Membrane A)

[0083] The substrate, a 9 inch ×9 inch piece of high densitypolyethylene (obtained from 3 M, product ID #43-9100-6770-1, 81%porosity, approximately 50 micron) was clamped in a frame and immersedin a N,N-dimethylformamide solution (7% w/w) of a sulfonated copolymerof α,β,β-tri-fluorostyrene and m-trifluoromethyl-α,β,β-tri-fluorostyrene(equivalent weight 384 g/mol) in a glass container. The container wascovered to exclude moisture and particulate contaminants. After 1 hourexcess polymer solution was removed and the transparent, wettedsubstrate was placed to dry in a chamber at approximately 2% relativehumidity, at 50° C. After approximately 3 hours the dry compositemembrane, now opaque, was a mechanically strong flexible film. Onimmersion in 1 M hydrochloric acid (to ensure protonation of all thesulfonic acid moieties), and subsequent washing with deionized water,the composite membrane once again became transparent. The wet compositemembrane (50-60 micron thick) was also strong and flexible.

[0084] EXAMPLE 2

[0085] Porous Polyethylene Impregnated with SulfonatedPoly-α,β,β-trifluorostyrene

[0086] (Composite Membrane B)

[0087] The substrate, a 10 inch ×10 inch piece of high densitypolyethylene (from 3 M, product ID #43-9100-6770-1, 81% porosity, 50micron) was clamped in a frame and immersed in a N,N-dimethylformamidesolution (7% w/w) of a sulfonated polymer of α,β,β-trifluorostyrene(equivalent weight 430 g/mol) in a glass container. The container wascovered to exclude moisture and particulate contaminants. After 2 hoursexcess polymer solution was removed and the transparent, wettedsubstrate was placed to dry in a chamber at approximately 2% relativehumidity, at 50° C. After approximately 3 hours the dry compositemembrane, now opaque, was a mechanically strong flexible film, incontrast to the analogous unsupported membrane which would be extremelyfragile in the dry state. On immersion in 1 M hydrochloric acid (toensure protonation of all the sulfonic acid moieties), and subsequentwashing with deionized water, the composite membrane once again becametransparent. The wet composite membrane (approximately 100 micron thick)was also strong and flexible.

[0088] EXAMPLE 3

[0089] Porous Polyethylene Impregnated with a Copolymer ofα,β,β-trifluorostyrene, m-trifluoromethyl-α,β,β-trifluorostyrene andp-sulfonyl fluoride-α,β,β-trifluorostyrene, and Subsequent Hydrolysis

[0090] (Composite Membrane C)

[0091] The substrate, a 10 inch ×10 inch piece of high densitypolyethylene (from 3 M, product ID #43-9100-6770-1, 81% porosity,approximately 50 micron) was clamped in a frame and immersed in aN,N-dimethylformamide solution (5% w/w) of a copolymer ofα,β,β-trifluorostyrene, m-trifluoromethyl-α,β,β-trifluorostyrene andp-sulfonyl fluoride-α,β,β-trifluorostyrene (equivalent weight 480 g/molafter hydrolysis) in a glass container. The container was covered toexclude moisture and particulate contaminants. After 2 hours excesspolymer solution was removed and the transparent, wetted substrate wasplaced to dry in a chamber at approximately 2% relative humidity, at 50°C. After approximately 3 hours the dry composite membrane was amechanically strong flexible film. The sulfonyl fluoride moieties werehydrolyzed by treatment of the composite membrane with potassiumhydroxide solution (approximately 6% w/w, in 5:1 w/wwater:1-methoxy-2-propanol) at 60° C. (see U.S. Pat. No. 5,310,765). Thecomposite membrane was then immersed in 1 M hydrochloric acid to ensureprotonation of all the sulfonic acid moieties in the composite membrane,and subsequently washed with deionized water. The wet, hydrolyzedcomposite membrane (50-70 micron thick) was also strong and flexible.

[0092] EXAMPLE 4

[0093] Expanded Polytetrafluoroethylene Impregnated with a SulfonatedCopolymer of α,β,β-trifluorostyrene andm-trifluoromethyl-α,β,β-trifluorostyrene

[0094] (Composite Membrane D)

[0095] The substrate, an 8 inch ×8 inch piece of expandedpolytetrafluoroethylene (Tetratex® obtained from Tetratec Corporation,80-90% porosity, approximately 38 micron, 0.45 micron pore size) wasclamped in a frame and immersed in a methanol/propan-2-ol (3:1) solution(approximately 5% w/v) of a sulfonated copolymer ofα,β,β-trifluorostyrene and m-trifluoromethyl-α,β,β-trifluorostyrene(equivalent weight 412 g/mol) in a glass container. The container wascovered to exclude moisture and particulate contaminants. After 18 hoursexcess polymer solution was removed and the transparent, wettedsubstrate was placed to dry in a chamber at approximately 2% relativehumidity, at 50° C. After approximately 1.5 hours the dry compositemembrane, now opaque, was a mechanically strong flexible film. Onimmersion in 1 M hydrochloric acid (to ensure protonation of all thesulfonic acid moieties), and subsequent washing with deionized water,the composite membrane once again became transparent. The wet compositemembrane (50-60 micron thick) was also strong and flexible.

[0096] EXAMPLE 5

[0097] Expanded Polytetrafluoroethylene Impregnated with a SulfonatedCopolymer of α,β,β-trifluorostyrene andm-trifluoromethyl-α,β,β-trifluorostyrene

[0098] (Composite Membrane E)

[0099] The composite membrane was prepared as described in Example 4,using a sulfonated copolymer of α,β,β-trifluorostyrene andm-trifluoromethyl-α,β,β-trifluorostyrene with a lower equivalent weight(362 g/mol) and impregnating the substrate for 30 minutes. The resultingdry composite membrane was a mechanically strong flexible film, incontrast to the analogous unsupported membrane which, at this lowequivalent weight, is extremely fragile and readily reduced to a powderon handling. The wet composite membrane (25-40 micron thick) was alsostrong and flexible, again in contrast to the unsupported membrane whichis fragile and dimensionally unstable, and is therefore of limited usein electrochemical fuel cells.

[0100] EXAMPLE 6

[0101] Each of the composite membranes prepared as described above wasbonded to two catalyzed carbon fiber paper electrodes at roomtemperature under 7,500 pounds of pressure. Each membrane electrodeassembly was tested in a Ballard Mark IV single cell fuel cell (see U.S.Pat. Nos. 4,988,583; 5,108,849; 5,170,124; 5,176,966 and 5,200,278; allincorporated herein by reference in their entirety). The followingoperating conditions applied to the fuel cell in which the membraneswere tested:

[0102] Temperature: 70° C.;

[0103] Reactant inlet pressure: 24 psi for both air and hydrogen;

[0104] Reactant stoichiometries: 2.0 air and 1.15 hydrogen.

[0105] The membrane electrode assemblies incorporating the compositemembranes were tested for 200-1400 hours, depending on availability oftesting equipment.

[0106] FIGS. 1-5 are polarization plots of voltage as a function ofcurrent density for composite membranes A-E, respectively, employed inmembrane electrode assemblies in the electrochemical fuel cell. The datais comparable to data reported for unsupported membranes in related U.S.Pat. No. 5,422,411.

[0107] While particular elements, embodiments and applications of thepresent invention have been shown and described, it will be understood,of course, that the invention is not limited thereto since modificationsmay be made by those skilled in the art, particularly in light of theforegoing teachings. It is therefore contemplated by the appended claimsto cover such modifications as incorporate those features which comewithin the scope of the invention.

What is claimed is:
 1. A membrane electrode assembly comprising first and second electrodes and a composite membrane interposed therebetween, wherein the composite membrane comprises a porous substrate impregnated with a polymeric composition comprising polymerized α,β,β-trifluorostyrene monomeric units. 